Light Emitting diodes (hereafter, “LED,” or “LEDs”) can generate visible or non-visible light in a specific region of the light spectrum. The light output from an LED may be, for example, light in the blue, red, green, non-visible ultra-violet (UV), and/or near-UV light, depending on the material composition of the LED. When it is desired to construct an LED light source that produces an emission color different from the output color of the LED, it is known to convert the light output from an LED having a first wavelength or wavelength range (i.e., “primary light” or “excitation light”) to light having a second wavelength or wavelength range (i.e, “secondary light” or “emission light”) using photoluminescence.
Photoluminescence generally involves absorbing higher energy primary light with a wavelength-conversion material such as a phosphor or mixture of phosphors. Absorption of the primary light can excite the wavelength-conversion material to a higher energy state. When the wavelength-conversion material returns to a lower energy state, it emits secondary light, generally of a different and longer wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light can depend on the type of wavelength-conversion material used. As such, secondary light of a desired wavelength/wavelength range may be attained by proper selection of wavelength-conversion material. This process may be understood as “wavelength down conversion,” and an LED combined with a wavelength-conversion structure that includes wavelength-conversion material, such as phosphor, to produce secondary light, may be described as a “phosphor-converted LED” or “wavelength-converted LED.”
In a known configuration, an LED die such as a III-nitride die is positioned in a reflector cup package and a volume, and a conformal layer or thin film of or including wavelength-conversion material is deposited directly on the surface of the die. In another known configuration, the wavelength-conversion material may be provided in a solid, self-supporting flat structure, such as a ceramic plate, a filled polymer material (e.g., polymer matrix filled with phosphor particles, a single crystal plate or thin film structure. Such a plate may be referred to herein as a “wavelength-conversion plate.” In any case, structures containing wavelength-conversion material that are attached directly to the LED die, e.g. by coating, wafer bonding, sintering, gluing, etc. may be understood as being in a “chip level conversion” configuration or “CLC.” Alternatively, structures containing wavelength-conversion material that are positioned remotely from the LED die may be understood as being in a “remote conversion” configuration.
Polymer-phosphor composites have been proposed for use as wavelength-conversion structures in LED light sources. Such composites generally include particles of one or more wavelength-conversion materials such as a phosphor (e.g., yellow phosphor, green phosphor, red phosphor, etc.) that are dispersed or otherwise loaded in a polymeric matrix, such as silicone. In many instances, the polymers used in such composites have a refractive index ranging from 1.3 to 1.6, whereas wavelength-converting phosphor fillers have a refractive index ranging from 1.7 to 1.9. The difference in refractive index between the polymeric matrix and the filler can cause significant scattering of light within a polymer-phosphor composite, and may result in the loss of light at the phosphor-polymer interfaces due to high total internal reflection (TIR). As a result, the optical transparency of such polymer-phosphor composites may be low, and may render such composites undesirable for use as a wavelength-conversion material in a lighting system.
In addition, heat (e.g., Stokes heat) is generated when a wavelength conversion material such as a phosphor converts primary light to secondary light. Due to their relatively low thermal conductivity, polymer-phosphor composites may not sufficiently remove heat generated by the phosphor particles during the conversion process. This may cause the phosphor particles to overheat, potentially lowering their efficacy. Excess heat may also contribute to the degradation of the phosphor particles and/or the polymeric matrix material.
Accordingly, while known polymer-phosphor composites may be useful for some applications, their usefulness in lighting applications may be limited due to their low optical transparency and/or low thermal conductivity.
For a thorough understanding of the present disclosure, reference should be made to the following detailed description, including the appended claims, in connection with the above-described drawings. Although the present disclosure is described in connection with exemplary embodiments, the disclosure is not intended to be limited to the specific forms set forth herein. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient. Also, it should be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting, except as otherwise expressly indicated.